The bandwidths and dynamic ranges of transmission signals are widened with the increasing speed of wireless communication in recent years. In order to minimize degradation of signal quality in such a situation, it is desirable for power amplifiers to have high linearity. In addition, it is also desirable to provide the power amplifiers that operate at high power conversion efficiency in term of reduction in size of the apparatuses, reduction in operational cost, environmental issues, and so on.
In general power amplifiers, the linearity is inconsistent with the power conversion efficiency. For example, operating the power amplifiers in linear areas backed off from saturated power allows an occurrence of out-of-band distortion to be reduced. However, the power conversion efficiency is considerably reduced to increase the power consumption of the power amplifiers in this case. Accordingly, in order to achieve both the linearity and the power conversion efficiency, the power amplifiers are operated in non-linear areas where the power conversion efficiency is high to keep the linearity by using distortion compensation for removing non-linear distortion occurring when the power amplifiers are operated in the non-linear areas. A pre-distortion method (hereinafter referred to as a “PD method”), which is a mode of the distortion compensation, is a technique in which a transmission signal is multiplied by an inverse characteristic of the non-linear distortion of the power amplifier in advance to improve the linearity in the output from the power amplifier.
An exemplary power amplifier apparatus in related art using the PD method will now be described. FIG. 18 illustrates an exemplary configuration of the power amplifier apparatus in the related art adopting the PD method. Referring to FIG. 18, the power amplifier apparatus includes a transmission signal generator 11, a distortion compensator 12, a digital-to-analog (D/A) converter 13, a quadrature modulator (QMOD) 14, an oscillator 15, a power amplifier 16, and an antenna 17. The power amplifier apparatus also includes a quadrature demodulator (QDEM) 18, an oscillator 19, an analog-to-digital (A/D) converter 20, a subtractor 21, and a coefficient generator 22.
A transmission signal generated by the transmission signal generator 11 is supplied to the distortion compensator 12 where the transmission signal is multiplied by a distortion compensation coefficient. The signal resulting from multiplication of the transmission signal by the distortion compensation coefficient is generated as a pre-distortion signal. The pre-distortion signal includes distortion components having the inverse characteristic of the non-linear distortion of the power amplifier 16. The pre-distortion signal is converted into an analog signal by the D/A converter 13, is up-converted by the QMOD 14 and the oscillator 15, and is supplied to the power amplifier 16. An output signal from the power amplifier 16, from which the non-linear distortion is removed by the pre-distortion process, is branched into two by, for example, a directional coupler. One of the output signals resulting from the branching is transmitted to an external reception apparatus via the antenna 17. The other of the output signals is down-converted by the QDEM 18 and the oscillator 19 and is converted into a digital signal by the A/D converter 20 to generate a feedback signal. The feedback signal and the transmission signal generated by the transmission signal generator 11 are supplied to the subtractor 21. An error signal, which is the difference between the feedback signal and the transmission signal, is calculated by the subtractor 21, the distortion compensation coefficient is generated by the coefficient generator 22 so that the error signal is minimized, and the generated distortion compensation coefficient is supplied to the distortion compensator 12.
For example, refer to International Publication No. WO/2001/008320.